Process for compacting metal powder

ABSTRACT

A PROCESS FOR PRODUCING BILLETS OF METALS AND METAL ALLOYS EMPLOYING POWDER METALLURGICAL TECHNIQUES WHEREIN THE RESULTANRT CONSOLIDATED POWDER MASSES ARE CHARACTERIZED AS HAVING A WROUGHT-TYPE GRAIN STRUCTURE POSSESSED OF EXCELLENT HIGH TEMPERATURE PHYSICAL PROPPERTIES. THE PROCESS RELIES ON THE USE OF TWO SEQUENTIALLYPHASED COMPACTION STEPS, WHEREBY THE METAL POWDER PACKED WITHIN A SEALED DUCTILE CONTAINER IS FIRST HOT ISOSTATICALLLY PRESSED TO A DENSITY OF AT LEAST ABOUT 90% OF THEORETICAL DENSITY AND THEREAFTER IS EXTRUDED AT AN ELVATED TEMPERATURE AT AN EXTRUSION RATIO GREATER THAN ABOUT 2:1 TO PRODUCE A BILLET WHICH APPRAOCHES 100% THEORETICAL DENSITY AND IS OF SUBSTANTIALLY UNIFORM GRAIN STRUCTURE THROUGHOUT.

July16,1974 J.W-SMYTHE ETAL 3,824,097

PROCESS FOR COMPACTING METAL POWDER Filed Dec. 19, 1972 United States Patent O PROCESS FOR COMPACTIN G METAL POWDER John W. Smythe and Steven H. Reichman, Ann Arbor,

and Don M. Weaver, Birmingham, Mich., assignors to Federal-Mogul Corporation, Southfield, Mich.

Filed Dec. 19, 1972, Ser. No. 316,490 Int. Cl. B22f 3/14 US. Cl. 75-226 Claims ABSTRACT OF THE DISCLOSURE A process for producing billets of metals and metal alloys employing powder metallurgical techniques wherein the resultant consolidated powder masses are characterized as having a wrought-type grain structure possessed of excellent high temperature physical properties. The process relies on the use of two sequentiallyphased compaction steps, whereby the metal powder packed within a sealed ductile container is first hot isostatically pressed to a density of at least about 90% of theoretical density and thereafter is extruded at an elevated temperature at an extrusion ratio greater than about 2:1 to produce a billet which approaches 100% theoretical density and is of substantially uniform grain structure throughout.

BACKGROUND OF THE INVENTION Various metal alloys are possessed of a metallurgical structure in the as-cast condition and are possessed of physical properties at both room and elevated temperatures that makes them very difiicult to post form to the desired shape utilizing conventional post-forming techniques. Typical of such metal alloys are the so-called nickel-based superalloys which can be characterized as normally having carbide strengthening and gamma prime strengthening in their cast and wrought forms. Such superalloys contain relatively large quantities of second phase gamma prime and complex carbides in a nickel-chromium gamma matrix. The presence of these gamma prime and complex carbides are responsible for the excellent high temperature physical properties of such alloys but also renders cast ingots of these alloys difiicult to post form and susceptible to macrosegregations resulting in billets of nonuniform microstructure and less than optimum physical properties.

The foregoing problems have been overcome to a large extent by employing powder metallurgical techniques wherein the metal alloys are first reduced to a powder state and thereafter are compacted at an elevated temperature employing either one of a variety of compaction techniques including hot pressing, hot isostatic pressing, extrusion, forging, explosive compaction, or the like. Conventionally, the metal powder is confined in a suitable ductile container within which it is heated to avoid contamination thereof and whereafter the container and its powder contents are densified under pressure in ac cordance with one of the foregoing compaction techniques. Typical of such compaction processes are those described in US. Pat. No. 3,655,458, granted Apr. 11, 1972, for Process for Making Nickel-Based Superalloys, and US. Pat. No. 3,671,230, granted June 20, 1972, for Method of Making Superalloys, which are assigned to the same assignee as the present invention. In accordance with the methods as described in the aforementioned United States patents, the metal alloy in the form of a powder of controlled particle size and composition is loosely packed wtihin a ductile container which is thereafter sealedto avoid contamination of the powder particles therein and whereafter the powder is heated and densified by hot pressing or extrusion.

It has been found that the compaction of metal alloy powders by extrusion of a ductile container filled with said powder comprises a particularly satisfactory technique for forming relatively large, elongated billets from which rotors, shafts and hubs can be fabricated for use in the hot sections of gas turbine engines and the like. During the extrusion operation, the loosely packed and confined powder is densified from an original density of about 60% to about theoretical density accom panied by a substantial reduction in the cross sectional area of the ductile container as well as an appreciable elongation thereof. In order to achieve a compaction whereby densities approaching 100% theoretical density are attained, it has been found necessary to employ extrusion ratios of at least about 6:1 to as high as about 10:1. While mild steel containers can be employed for confining the powder during the preheating and extrusion operation, the susceptibility of such mild steel containers to oxidation in air at the elevated temperatures to which they are heated has occasioned an increased use of the higher cost stainless steel containers which are more resistant to such oxidation attack and are of superior high temperature physical properties. Even when using such stainless steel containers, an occasional fracture or rupture of the container occurs during the extrusion operation exposing its metallic powder contents to contamination resulting in an extruded billet of inferior physical properties.

The extrusion operation is also accompanied by an initial densification of the powder in the container by inward deformation of the rearward end thereof prior to initiation of the extrusion through the die orifice. This frequently is accompanied by a wrinkling or buckling of the container walls such that the resultant billet must be machined to an appreciable depth in order to remove the last traces of the container wall therefrom which comprises a costly operation and also is wasteful of the high cost superalloy material. It has also been observed that in some instances the densification of the powder by the extrusion process has resulted in billets of nonuniform density which incorporate local porous sections and areas of less than 100% theoretical density. The presence of such imperfections renders the compacted billet unsuitable for fabricating components requiring extremely high physical properties at the elevated service temperatures to which they are to be subjected.

The foregoing problems associated with prior art compaction techniques are overcome in accordance with the practice of the process of the present invention wherein a ductile container filled with the metal powder to be compacted is subjected to an initial compaction step in which a preliminary densification of the powder is effected, as well as a reduction in the size of the container, followed thereafter by the conventional extrusion operation which can be performed at less drastic extrusion conditions.

SUMMARY OF THE INVENTION The benefits and advantages of the present invention are achieved by a process in which the metal, metal alloy, intermetallic and/or nonmetallic material is first reduced to a finely particulated powder form having a particle size usually less than about 250 microns and thereafter the powder is loosely packed at a density of usually 60% to 70% of theoretical density within the interior of a fluid impermeable ductile container. The container is preferably evacuated and thereafter sealed to avoid any contamination of the powder contents. The sealed container and the powder contents thereof are subsequently heated to an elevated temperature, whereafter the container is subjected to an exteriorly applied isostatic fluid pressure of a magnitude usually above about 1000 p.s.i. for a period of time sufiicient to effect a reduction in the size of the container and a compaction of its powder contents to a density greater than about 90% and preferably to a density of greater than about 98% of theoretical density. The preliminarily compacted powder mass and container at an elevated temperature is thereafter passed through an extrusion die in a longitudinally oriented direction at an extrusion ratio of at least about 2:1 and in a manner to effect an extrusion and an elongation of the container an a densification of the powder therein into a coherent mass approaching substantially 100% theoretical density. Upon cooling of the composite extruded billet, the container encapsulating the periphery thereof is removed such as by machining or the like.

Additional advantages and benefits of the present invcntion will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal cross-sectional view of a typical container filled with metallic powder which is sealed therein preparatory to compaction in accordance with the practice of the present invention;

FIG. 2 is a longitudinal vertical sectional view of the container and powder contents thereof after completion of the preliminary hot isostatic compaction step; and

FIG. 3 is a longitudinal vertical sectional view of the final extruded billet produced by the hot extrusion of the preliminary compacted powder mass and container of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the process comprising the present invention is described herein as being particularly applicable for the compaction of so-called nickel-based superalloys, it will be understood that the process is equally applicable for the consolidation of powder masses in which the particles are comprised of metals and metal alloys other than superalloys as well as powders comprised of intermetallic compounds, nonmetallic compounds, as well as mixtures of the foregoing. Typical of the intermetallic compounds are boron carbide, tungsten carbide, titanium carbide, tantalum carbide, uranium oxide, intermetallic compounds of uranium, etc. The process, accordingly, can be advantageously employed for producing compacted billets of powdered materials of the types which are characterized as being difficult to post form in an as-cast conditions, and/ or which are difficult to machine, and/or which are of relatively high cost and/or which, in some instances, undergo madrosegregation during the post forming of solid ingots thereof, thereby preventing the attainment of optimum physical properties.

A representation of typical nominal composition of nickel and iron based alloys which can be advantageously processed in accordance with the present invention is set forth in Table 1.

The reduction of metal alloys of the types enumerated in Table 1 can be conveniently and efficiently achieved by any one of the variety of known processing techniques of which the microcasting of a molten mass of the metal alloy by gas atomization constitutes a preferred method. The gas atomization of a molten mass of metal can conveniently be achieved by employing apparatuses of the types described in U.S. Pat. No. 3,253,783, which is assigned to the same assignee as the present invention and the teachings of which are incorporated herein by reference. Because of the deleterious effects of oxides on the physical properties of the compacted powder mass, and the reactivity of many alloying agents with oxygen, it is usually preferred to perform the gas atomization of the molten alloy and the solidification and collection of the powder particles in neutral environments devoid of any reactive constituents.

The permissible oxygen content of the powder is in part determined by the specific composition of the powder material and the deleterious effects of oxides on the resultant physical properties of the compacted powder mass. In the case of nickel-based superalloys which usually contain aluminum and titanium as alloying constituents, a high degree of precaution is necessary because of the propensity of these two elements to react with oxygen particularly at the high temperatures at which the microcasting is performed. The contamination of such superalloy powders with oxides in an amount above about 200 p.p.rn. has also been found in some instances to significantly detract from the ultimate physical properties of the compacted billet and it is for this reason the microcasting operation is usually carried out in a substantially dry inert atmosphere so that the superalloy powder has an oxygen content of less than about 100 p.p.m. Particularly satisfactory results have been obtained by employing helium or commercially available argon gases which contain only minimal amounts of conventional impurities as the atomizing medium and also the atmosphere in which the solidification, collection and classification of the microcast powder is effected.

Regardless of whether the powder is produced in ac cordance with the aforementioned inert gas atomization technique, or by mechanical comminution, airless spraying or the like, it is usually preferred that the individual powder particles are of substantially the same or similar alloy chemistry. In the case of mixtures of two or more dissimilar powders, a preliminary blending is effected to assure a substantially uniform distribution of the particles throughout the mass. The particular shape of the individual powder particles is not critical, although superalloy powders derived from microcasting are conventionally of a spherical configuration. The powder particles are also selected so as to be of an average size up to about 250 microns down to a size as small as about one micron. For superalloy powders, it is preferred that the average particle size be controlled within a range of from about 150 microns to about 10 microns with the particles being ran- TABLE 1.NOMINAL COMPOSITIONS OF SOME NICKEL-BASED SUPERALLOYS Percent by weight Al Ti Mo W C 0 Ch B domly distributed over the aforementioned range, thereby providing for a maximum packing density of the powder within the container. Loose packing densities of the powder in the container will usually range from about 60% to about 70% of theoretical density and this can be maximized for any particular powder configuration and size by further subjecting the container and its powder contents to sonic or supersonic vibrations during the filling operation.

Referring now in detail to the drawing, a device for use in accordance with the process for compaction of metallic powers comprises an elongated container 10, as shown in FIG. 1, which is composed of a ductile fluid-impervious material, of which metals such as mild steel or stainless steel are typical. The container comprises a front end plate 12, a thin-walled body section 14 and a rearward end plate 16 which are joined together by welding and which, in combination, define an elongated internal chamber 18. The chamber 18, in the exemplary embodiment shown, is of a right cylindrical configuration. The chamber 18 is loosely filled and packed with a metal powder 20 which is introduced through a deformable tube 22 secured such as by means of welding to the outer face of the end plate and in alignment with a port 24. The filling operation is preferably performed under vacuum. At the completion of the filling and vibration operation, the deformable tube 22 can be satisfactorily crimped, such as indicated at 26, and further welded, if desired, to assure the formation of a fluid-tight seal.

As previously indicated, the two-step compaction operation enables the use of containers comprised of mild steel, such as AISI Type 1010 steel, particularly when the hot isostatic compaction is performed employing a pressurized inert gas, such as argon, which prevents oxidation attack of the surface of the container at the elevated temperatures employed. This provides for a substantial cost saving over the use of stainless steel type containers, such as AISI Type 304 stainless steel, which constitutes a further advantage of the present process. It will be understood that alternative materials which are ductile and fluid impervious and which are possessed of physical strength properties at the temperatures employed similar to that of the mild and stainless steels can also be satisfactorily employed for fabricating the container 10.

In accordance with the two-stage compaction process, the filled container 10 containing the metallic powder loosely packed to a density usually ranging from about 60% to about 70% of theoretical density is placed within an autoclave in which it is adapted to be heated and subjected to an external pressure for a period of time sufiicient to effect a compaction of the metallic powder contents to a density of at least about 90% and prefer ably greater than about 98% of theoretical density. By employing high temperatures and pressures normally utilized in the hot isostatic compaction techniques known, a densification of the metallic powder in excess of 99% and approaching 100% theoretical density can be achieved within reasonable time periods.

The specific temperature and pressure employed will vary depending upon the specific composition, particle size and particle configuration of the powder within the container, as well as the conditions to which the compacted powder will be subjected during the subsequent hot extrusion operation. conventionally, the temperature to which the container and its powder contents are heated during the hot isostatic pressing step can range from about 1000" F. (eg. for aluminum and aluminum alloy powders) up to about 3000 F. (eg. for intermetallic compounds such as tungsten carbide). For superalloy powders, the temperature is usually controlled within about 1900" F. to about 2300 F. and preferably from about 2000 F. to about 2200 F. The pressure to which the container and its powder contents are subjected may range from as low as about 1000 p.s.i. to as high as is possible in consideration of the strength of the pressure vessel or autoclave within which the compaction is effected. More usually, pressures" of from about 5000 to about 15,000 p.s.i. are employed for most metal powders which, in combination with temperatures of from about 2000 to about 2200 F., provide for a substantially complete compaction of the powder within reasonable time periods of up to about "10 hours.

Due to the elevated temperatures utilized during the hot isostatic compaction, the pressure transmitting medium comprises a gas, of which any one or a variety of the inert gases can be employed for this purpose. Particularly satisfactory results are achieved utilizing argon of commercial quality as the fluid medium for app-lying pressure simultaneously and equally over the entire surface area of the container whereby the resultant compacted powder mass is of substantial uniform density throughout.

FIG. 2 is illustrative of a container 28 which has been reduced in size and containing a preliminarily compacted powder 30 which has been densified by the hot isostatic pressing technique. As noted, the container 28 is of a reduced length and diameter in comparison to the container 10 of FIG. 1. The reduction in size of the container is a direct function of the increase in density of the powder contents achieved during the hot isostatic pressing. Surprisingly, a substantially uniform increase in the wall thickness of the container is also experienced.

In accordance with the preferred practice, the container 28 and the preliminarily densified powder 30, while still at an elevated temperature, are directly transferred to the next extrusion step in which the container is forced through an extrusion die of reduced cross sectional area in a manner so as to effect a further densification and lateral forging of the individual powder particles, producing an elongated extruded billet indicated at 32 in FIG. 3 which is possessed of a wrought-type grain structure. The extrusion of the container is facilitated by the use of a tapered nose section 34 as shown in FIG. 2 which may be separate or can be afiixed to one end of the container prior to extrusion. In the extrusion step, the container 28 is oriented with its longitudinal axis aligned with the axis of the extrusion die and with the tapered nose section 34 positioned adjacent to the die orifice. The extrusion step is carried out at an extrusion ratio greater than about 2:1 and preferably at extrusion ratios greater than about 3:1 up to as high as about 10:1. The extrusion ratio, i.e., the original cross sectional area divided by the final cross sectional area, is selected so as to produce a billet which is of substantially theoretical density and which is possessed of the desired wroughttype grain structure. The specific size of the extrusion orifice is controlled so as to provide a resultant billet of a size suitable for fabricating the desired components therefrom. Because of the increased density of the powder provided by the hot isostatic pressing step, extrusion ratios can be employed which are substantially lower than those required for effecting a compaction of powder which is in a loosely-packed condition of about 60% to 70% of theoretical density.

The temperature of the preliminarily densified powder 30 prior to the extrusion step can vary within the temperature ranges previously described as being suitable for the hot isostatic compaction step. In the case of superalloy powders, extrusion temperatures of from about 1800 to about 2200 F. are preferred.

At the completion of the extrusion operation, the extruded billet 32, as shown in FIG. 3, is allowed to cool, whereafter the elongated body section, indicated at 36, is removed such as by machining or grinding from the periphery of the densified powder mass, indicated at 38, and the extruded nose section 40 and end plate similarly are removed, providing therewith an elongated billet of the desired cross sectional configuration.

While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages hereinabove set forth, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

What is claimed is:

1. The process for making consolidated billets from powder which comprises the steps of confining a nickelbase superalloy powder of an average particle size ranging from about one micron to about 250 microns which contains less than about 200 p.p.m. oxygen in a fluidimpermeable and sealed ductile container, heating said container and said powder therein to a first elevated temperature, subjecting the heated said container and powder to an exteriorly-applied isostatic fluid pressure for a period of time sufficient to effect a compaction of said container and said powder therein to a density of at least about 90% of theoretical density, further densifying said powder by passing the compacted said container and said powder at a second elevated temperature longitudinally through an extrusion die at an extrusion ratio of at least about 2:1 and in a manner to effect an extrusion and an elongation of said container and a compaction of said powder therein into a coherent mass approaching 10% theoretical density, and thereafter removing said container from the periphery of said mass.

2. The process as defined in claim 1, wherein said first temperature ranges from about 1000" F. to about 3000 F.

3. The process as defined in claim 1, wherein said first elevated temperature ranges from about 1900 F. to about 2300 -F.

4. The process as defined in claim 1, wherein said first temperature ranges from about 2000 F. to about 2200 F.

5. The process as defined in claim 1, wherein said ex teriorly-applied isostatic fluid pressure is of a magnitude sufficient to effect a compaction of said powder to a density of at least about 98% of theoretical density.

6. The process as defined in claim 1, wherein the exteriorly-applied isostatic fluid pressure is applied for a period of time sufiicient to effect a compaction of said powder to a density of at least about 99% of 100% theoretical density.

7. The process as defined in claim 1, wherein said powder comprises particles of an average size ranging from about 10 microns up to about microns.

8. The process as defined in claim 1, wherein said second elevated temperature ranges from about 1800 F. to about 2200 F.

9. The process as defined in claim 1, wherein said isostatic fluid pressure is greater than about 1000 p.s.i.

10. The process as defined in claim 1, wherein said isostatic fluid pressure ranges from about 5000 p.s.i. up to about 15,000 p.s.i.

References Cited UNITED STATES PATENTS 3,341,325 9/1967 Cloran .a 75-226 3,070,440 12/1962 Grant et al. 75226 3,070,439 12/ 1962 Grant et al. 75226 3,655,458 4/1972 Reichman 75226 OTHER REFERENCES Buiferd et al.: Metal Progress, April 1971, Vol. 99 No. 4, A.S.M. T5300 M587, pp. 68-71.

CARL D. QUARFORTH, Primary Examiner B. HUNT, Assistant Examiner US. Cl. X.R. 75200, 214, 221

' UNITE STATES PATENT OFFICE A CERTIFIGATE OF CORRECTION Paten t No. 3,324,097 Dated July 16, 1974 V 'G) John W. Smythe, Steven H. Reichman and Don M. Weaver It is certified that er ror apuears in the above-identified patent and that said Letters Patent are hereby cor'rected as shown below:

Column 1,' line 68, "wtihin" should be --:within-- Column 3, line 9, "an" should be --and-- Column 3, lino 4 8, "conditions" should be: --conditi on- I Columh 1 i: ne 51, "madrosegregatiou" should be Q --.-macrosegregation-- Column 7, line 22, Claim 1, "1.0%0 should be "100%" si hed and Sealed thi s 3rd day of Deoember 1974.

(SEAL) Attest: v McCOY M. GIBSONJR. c. MARSHQLL DANN' Attesting' Officer v ,Comissiqner of Patents FORM PO-105O (10-69) h UScOMM-DC wan-Pee i 0.5, GOVCIIIIINY II INTIIIG Oil'ltl I. OJ-l34 I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,824,097 Dated July 16, 1974 e g) John W. Smythe, Steven H. Reichrnan and Don M. Weaver It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 6 8,- "wtihin" should be within-- Column 3, line 9, "an" should be --and-- Column 3, line 48, "conditions" should be "condition-- Column 3, line 51, "madrosegregation" should be -macrosegregation-- j Column '7, line 22, Claim 1, "10%l should be -100%-- Signed and sealed this 3rd day of Deeember 1974.

(SEAL) Attest: I I

McCOY M. GIBSON JR. c. MARSHALL DANN' Attesting' Officer 4 Comigsiqner of Patents 

